It is almost required to release mutual engagement of racks at first during rekeying process of known rekeyable lock cylinder and reengage racks after rekeying, such as U.S. Pat. No. 6,860,131 “rekeying a lock assembly” discloses a simple tool used for pushing linkage mechanism within a lock to rekey. However, linkage mechanism of the foregoing lock cylinder lacks burglarproof in design when rekeying as well as the simple tool is easy to obtain, that might be subject to unauthorized rekeying operation resulting in security problem.